UMass Amherst CHEM 265 - Recrystallization (9 pages)

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Recrystallization



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Recrystallization

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Pages:
9
School:
University of Massachusetts Amherst
Course:
Chem 265 - Organic Chem-Maj

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Recrystallization 1 Recrystallization is the most convenient method for purifying organic compounds that are solids at room temperature Compounds obtained from natural sources or from reaction mixtures almost always contain impurities The impurities may include some combination of insoluble soluble and colored impurities To obtain a pure compound these impurities must be removed Each is removed in a separate step in the recrystallization procedure To understand the recrystallization process solubility behavior must first be considered It is often stated that like dissolves like More correctly it should be stated that compounds having similar structural features will be soluble in one another Some obvious structural features that may affect solubility include polarity and the ability to hydrogen bond For example a compound having just a few carbons and an alcohol functional group FG would be expected to be soluble in solvents that have a few carbons and an alcohol FG or in some other polar solvent and to be less soluble in nonpolar solvents Conversely an alkane would be expected to show the opposite solubility behavior In most cases though it is not as simple as this If for example a compound has lots of carbons and hydrogens 6 C s and just one alcohol group the solubility will be dominated more by the alkyl part of the molecule than by the alcohol part and the compound will show a solubility behavior more like that of an alkane For known compounds it is useful to consider the structure of the compound when choosing a recrystallization solvent An educated guess can save some time Usually however the structure of a compound may not be known so the solvent must be chosen by carrying out solubility tests The first part of this experiment involves carrying out solubility tests on known compounds Later on such solubility tests will be used to find a suitable recrystallization solvent for an unknown compound A compound usually exhibits one of three general solubility



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